As for the single color liquid crystal display device for coloring its background and display parts, several types have been proposed in the past.
A first type is a single color liquid crystal display device made up by providing a color polarizing film on the outer side of a liquid crystal cell thereof. This type has generally been in widespread use due to its simple constitution.
A second type is a single color liquid crystal display device wherein a dichroic pigment is mixed in nematic liquid crystals sealed in a liquid crystal cell thereof, and the dichroic pigment is caused to move together with the nematic liquid crystals by the action of nematic liquid crystal molecules, and this type is called a guest-host LCD.
However, with any of these conventional-type single-color liquid crystal display devices, colored characters and colored graphics produced by a dye or the dichroic pigment are displayed against a background in white, or white characters and white graphics are displayed against a background color produced by the dye or the dichroic pigment. Consequently, contrast declines. Furthermore, since the numbers of available dyes and dichroic pigments are limited, there is also a problem that the number of colors in which such liquid crystal display devices as described above can indicate display.
Accordingly, there has been proposed a third type of single color liquid crystal display device comprising a polarizing film, a 90.degree. twisted nematic (TN) liquid crystal cell, a retardation film for circularly polarizing film (a quarter-wavelength plate, 1/4.lambda. plate), a cholestric liquid crystal polymer sheet, and a light absorbing member.
Referring to FIG. 13, the display principle of the single color liquid crystal display device of this type is described hereinafter.
Shown in FIG. 13, the single color liquid crystal display device is made up of a polarizing film 8, a twist-aligned 90.degree. TN liquid crystal cell which is not shown, a retardation film 9 for circularly polarized light, a cholestric liquid crystal polymer sheet 10, and a light absorbing member 11.
In the left-hand part of the figure, an OFF-state indicating colored display is shown, wherein the polarizing film 8 and the retardation film 9 for circularly polarized light are disposed such that the transmission axis 8b of the polarizing film 8 is inclined 45.degree. clockwise toward the phase delay axis 9a of the retardation film 9 for circularly polarized light. As a result, linearly polarized light transmitted through the polarizing film 8 is turned into right-handed circularly polarized light after passing through the retardation film 9 for circularly polarized light.
In the cholestric liquid crystal polymer sheet 10, a twist direction 10a is right handed, and a twist pitch is close to a natural light wavelength, and accordingly, upon the right-handed circularly polarized light falling on the cholestric liquid crystal polymer sheet 10, light components in a scattering band width .DELTA..lambda. around a scattering center wavelength .lambda.c are reflected due to the selective scattering phenomenon. Transmitted light composed of light components in wavelength regions, other than the scattering band width .DELTA..lambda., is absorbed by the light absorbing member 11 made of a black paper or a black plastic sheet, whereupon a bright reflected color can be obtained.
If one defines n as the refractive index of a cholestric liquid crystal polymer, and P as the twist pitch of the cholestric liquid crystal polymer, the scattering center wavelength .lambda.c is given by the following formula: EQU .lambda.c=n.times.P
Hence, the cholestric liquid crystal polymer sheet 10 having various reflected colors can be obtained by adjusting the twist pitch P of the cholestric liquid crystal polymer.
Meanwhile, in the right-hand part of FIG. 13, an ON-state indicating black display is shown, wherein the polarizing film 8 and the retardation film 9 for circularly polarized light are disposed such that the transmission axis 8a of the polarizing film 8 is inclined 45.degree. counterclockwise toward the phase delay axis 9a of the retardation film 9 for circularly polarized light by rotating the transmission axis 8a of the polarizing film 8 through 90.degree., so that linearly polarized light after passing through the retardation film 9 for circularly polarized light is turned into left-handed circularly polarized light.
Accordingly, even if the left-handed circularly polarized light falls on the cholestric liquid crystal polymer sheet 10 having the twist direction 10a which is right handed, the selective scattering phenomenon does not occur. As a result, all components of the left-handed circularly polarized light are transmitted through the cholestric liquid crystal polymer sheet 10, and absorbed by the light absorbing member 11, thus indicating black display.
If a 90.degree. TN liquid crystal cell is installed between the polarizing film 8 and the retardation film 9 for circularly polarized light instead of rotating the transmission axis 8a of the polarizing film 8, this makes it possible to change by about 90.degree. the polarization direction of linearly polarized light falling on the retardation film 9 for circularly polarized light depending on whether or not a voltage is applied to the TN liquid crystal cell, so that a background part in the OFF-state and display parts in the ON-state can be controlled optionally. Consequently, a single color liquid crystal display device having such a constitution is able to display black characters against a colored background in a bright metallic tone.
Such related art as described above has been disclosed in Japanese Patent Laid-Open No. S52-5550 and Japanese Patent Laid-Open No. H6-230362. There has also been disclosed in, for example, Japanese Patent Laid-Open No. H6-230371, a similar liquid crystal display device comprising a first polarizing film, a TN liquid crystal cell, a second polarizing film, a retardation film for circularly polarized light, a cholestric liquid crystal polymer sheet, and a light absorbing member, wherein the second polarizing film is installed between the TN liquid crystal cell and the retardation film for circularly polarized light.
However, with these single color liquid crystal display devices using the cholestric liquid crystal polymer sheet, illumination by backlighting is not feasible because of use of an opaque light absorbing member made of a black paper, a black plastic sheet, or so forth, thus causing a problem that display can not be shown in a dark environment such as at night, and so forth.